In a typical hot rolling process for a polymeric sheet, the polymer being e.g. polyethylene-terephthalate (PET) or polyethylene (PE), the extrudate is subjected to successive rolling operations reducing its thickness. The resulting polymeric sheet and the rollers are typically maintained at temperatures below but near the melting point of the polymer during the process.
Due to rolling, directional elongation and also, sometimes, to additional stretching, the polymer sheet acquires an orientation, i.e. the molecular chains become preferentially oriented in the longitudinal direction (parallel to the sheet movement). On-line evaluation of such orientation would be useful for both quality and process control since the orientation directly affects the mechanical properties of the polymer sheet. Ideally, such methods should be non-contact so as to introduce no perturbation to the ongoing process.
Prior art methods and techniques for evaluating molecular orientation involve ultrasonic velocity anisotropy, thermal anisotropy, X-ray diffraction, optical birefringence and optical dichroism measurements. Another technique requiring a transmission configuration is small-angle light scattering, as described in U.S. Pat. No. 4,264,207 by Batyrev et al. This technique is, however, affected both by molecular and microstructural orientation, while also being very sensitive to directional surface roughness.
Ultrasonic velocity anisotropy requires transmitting of ultrasonic radiation in different directions within the polymer. This typically requires liquid contact of the transducer with the material.
Thermal anisotropy can be monitored without contact by heating a spot on the sheet surface by a focused laser beam and monitoring the ellipticity of the temperature distribution around the heated spot by a thermographic camera. However, the thermal response is slow due to the low thermal diffusivity of the polymer, while induced localized temperature gradients may interfere with the viscoelastic rolling process.
X-ray diffraction is often used to evaluate crystalline and oriented materials, but this technique is not appropriate to on-line applications because it is expensive, unsafe and very sensitive to positioning.
Optical birefringence, i.e. a difference in the refractive index values depending on the polarization of the incident light beam, is strongly related to molecular orientation. Birefringence can be measured for example with a set-up as shown in FIG. 1: a light beam polarized either parallel or perpendicular to the plane of the figure is totally reflected at the interface between a prism 11 and a polymer film 10 in good contact with the prism, up to a critical value of the angle of incidence at which some light starts leaking into the film 10. As the critical angle depends on the film's refractive index, and this index is different for the two polarizations of the light beam, one can evaluate the two refractive indices, and thus the birefringence, by measuring the critical angles corresponding to the two polarizations of the incident light beam. This technique requires a good contact, usually via a matching liquid, and therefore is inadequate for on-line applications.
Non-contact birefringence techniques are also possible, e.g. by monitoring the phase retardation induced by the insertion at different angles of a polymer film in a polarized interferometer, as described in U.S. Pat. Nos. 4,909,630 to Gawrisch et al. and 4,973,163 to Sakai et al, but this typically requires a clear and smooth, usually very thin, polymer film.
Similar to birefringence is the measurement of optical dichroism i.e. a difference in the material absorbance depending on the polarization of the light beam transmitted through the polymer film 12 as shown in FIG. 2. The atoms which make up a polymer molecule vibrate according to well defined normal modes, many of which are highly localized on particular chemical bonds or groups of bonds. Certain vibrational modes produce a fluctuation in dipole moment known as the transition moment, which has a specific direction with respect to the long chain of the polymer molecule. The absorption of electromagnetic radiation is determined by the angle between the transition moment and the electric field vector of the radiation. The absorption intensity of a particular mode is the greatest when the two are parallel and zero when they are perpendicular. This is the basis of optical dichroism. A preferential orientation of the polymer chains results in a preferential orientation of the transition moments and hence a preferential absorption of light polarized with its electric vector parallel to the transition moments.
The measurement is typically performed by spectropolarimetry: light of different polarizations transmitted through a film 12 is spectrally analyzed by a monochromator (see, e.g. U.S. Pat. No. 4,309,110 by Tumerman) or a spectrometer (not shown in FIG. 2); if the film has transition moments preferentially aligned, say, in the plane of the figure, light polarized parallel to the plane of the figure will be preferentially absorbed. The corresponding spectral absorption band will thus be very strong with this polarization, while being much weaker when the incident light beam is polarized perpendicular to the plane of the figure. Again, this typically requires a clear and usually very thin polymer film (whose thickness must be of the order of the light penetration depth on the spectral absorption band--typically 1 to 50 micrometers), which is not the case for the typically 1 mm thick sheet in question.